In supercharged engines, since the supercharge pressure increases in proportion to the rotating speed of the engine, if the supercharge pressure increases too much, the supercharger or the internal combustion engine can be damaged. Therefore, for example, the durability of the engine is maintained by preventing the supercharge pressure from exceeding a preset value by way of a control apparatus for the waste gate system or the like, in which when the supercharge pressure of the intake passage downstream of the supercharger compressor exceeds a set value, a bypass which is provided so as to bypass the supercharger turbine is opened to drain a part of the exhaust air and thereby control the supercharge pressure.
On one hand, in conventional apparatuses of the waste gate system, if the supercharge pressure is controlled to become constant in order to satisfy the durability of the engine, then in the case where high engine power is temporarily required as in an accelerating operation, the required power performance is not achieved, so that there is a drawback such that the desired acceleration performance cannot be satisfied. Therefore, an apparatus which can control the supercharge pressure at a few stages has been proposed. However, in a conventional apparatus, as shown in FIG. 4, the supercharge pressure (indicated by an alternate long-and-short dash line) in the acceleration operating state of the engine is merely set higher than the supercharge pressure (indicated by a dotted line in FIG. 4) in the stable operating state of the engine, and the supercharge pressure before the interception point is not made high. Consequently, improvement in acceleration performance of the engine in the low-speed rotation range cannot be expected.
In addition, among such apparatuses, for instance, as disclosed in Japanese Patent Unexamined Publication No. 29728/1984, there is an apparatus in which a solenoid valve for allowing the operating supercharge pressure to be communicated with or shut off from the atmosphere is provided through a throttle in the way of the passage to lead the operating supercharge pressure into an actuator and thereby control the supercharge pressure at a few stages. However, this apparatus has a drawback in that the efficiency of the supercharger compressor deteriorates since a part of the supercharge pressure leaks.